This invention relates to a method for further purifying hydrogen peroxide containing only small amounts of dissolved inorganic or organic impurities, in order to prepare highly purified hydrogen peroxide suitable for microelectronics uses which require very high purity.
The production of ever more highly integrated circuits for microelectronics requires the preparation of high-performance chips. Although a few years ago storage capacities of 256K were still considered satisfactory, today the one-megabit chip has long held a place in integrated circuits, and the production of microchips with a storage capacity of as much as four megabits has already been undertaken. Rapid progress in microelectronics, especially with the aim of producing 1 and 64 megabit chips, not only calls for high-purity silicon wafers with impurities of less than 1 ppb (1 part per billion parts of silicon) but also requires that the chemicals needed for these super-integrated chips satisfy very stringent purity requirements. Although the production of one-megabit chips tolerates a chemical purity somewhere in the low ppm range (1 part per million), the production of four-megabit and sixteen-megabit chips requires chemical standards with maximum impurity contents in the range of less than 10 ppb.
One of the key chemicals used in chip manufacture which must satisfy these very high purity requirements is hydrogen peroxide. Since hydrogen peroxide, however, is prepared almost exclusively by the anthraquinone process and is ordinarily purified and concentrated by rectification in columns of aluminum or high grade steel, it does not have the required purity. Due to contact with the metal parts of the apparatus, the distillate is contaminated with metals, particularly with aluminum. Besides, hydrogen peroxide manufactured and purified in this manner contains process residues of organic carbon compounds ("organic C"), such as solvents (alcohols, ketones, aliphatic hydrocarbons, acids) and anthraquinone derivatives. For use in microelectronics the hydrogen peroxide must therefore be subjected to an effective post-treatment to decrease its content of cations, anions and carbon to the required degree of purity.
The known purification of hydrogen peroxide solutions by distillation alone does not, however, achieve the necessary purity of the hydrogen peroxide with reference to metal impurities and carbon. For example, distillation fails to eliminate organic carbon from hydrogen peroxide due to the fact that the hydrogen peroxide made by the anthraquinone process contains highly volatile or steam-vaporizable organic carbon compounds. The content of, for example, dissolved organic carbon in the hydrogen peroxide can quite easily exist at levels of up to 150 mg/l. Metal ion and carbon impurities in hydrogen peroxide are especially disturbing in the manufacture of microchips, and the more highly integrated the chips are to be, the more critical is the effect of impurities. Therefore, there has been no lack of attempts in the prior art to remove the impurities from hydrogen peroxide by post-treatment with cation and/or anion exchangers.
Among the ion exchange materials proposed in the prior art for this purpose are aromatic hydrocarbon cation exchange resins sulfonated on the core for the removal of cations, and aromatic hydrocarbon anion exchange resins containing tertiary amino or quaternary ammonium groups or pyridine rings for the removal of anions. Due to the functional groups contained in these ion exchange resins, the ion exchange resins are often so sensitive to oxidation that, when hydrogen peroxide is purified with these ion exchange resins it is necessary to operate at relatively low temperatures of about 0.degree. C. and to use special apparatus.
The high sensitivity of the ion exchange resins to oxidation is attributable to the fact that, in the presence of heavy metals, such as Fe or Cu etc., hydrogen peroxide can form dangerous hydroxyl radicals which oxidatively attack the carbon skeleton of the ion exchanger and can form easily degradable epoxides or hydroperoxides with it. The epoxides or hydroperoxides that form can not only decompose explosively but can even detonate under certain circumstances. The use of cation or anion exchangers for purifying hydrogen peroxide is therefore questionable for safety reasons and requires special care, since dangerous decomposition of hydrogen peroxide can occur in the ion exchanger.